Relaxed silicon germanium platform for high speed CMOS electronics and high speed analog circuits

ABSTRACT

Structures and methods for fabricating high speed digital, analog, and combined digital/analog systems using planarized relaxed SiGe as the materials platform. The relaxed SiGe allows for a plethora of strained Si layers that possess enhanced electronic properties. By allowing the MOSFET channel to be either at the surface or buried, one can create high-speed digital and/or analog circuits. The planarization before the device epitaxial layers are deposited ensures a flat surface for state-of-the-art lithography.

PRIORITY INFORMATION

This application claims priority from provisional application Ser. No.60/273,112 filed Mar. 2, 2001.

BACKGROUND OF THE INVENTION

The invention relates to the field of relaxed SiGe platforms for highspeed CMOS electronics and high speed analog circuits.

Si CMOS as a platform for digital integrated circuits has progressedpredictably through the industry roadmap. The progress is createdthrough device miniaturization, leading to higher performance, greaterreliability, and lower cost. However, new bottlenecks in data flow areappearing as the interconnection hierarchy is expanded. Although digitalintegrated circuits have progressed at unprecedented rates, analogcircuitry has hardly progressed at all. Furthermore, it appears that inthe near future, serious economic and technological issues will confrontthe progress of digital integrated circuits.

The digital and communication chip markets need an enhancement to SiCMOS and the maturing roadmap. One promising candidate material thatimproves digital integrated circuit technology and introduces new analogintegrated circuit possibilities is relaxed SiGe material on Sisubstrates. Relaxed SiGe alloys on Si can have thin layers of Sideposited on them, creating tension in the thin Si layers. Tensile Silayers have many advantageous properties for the basic device inintegrated circuits, the metal-oxide field effect transistor (MOSFET).First, placing Si in tension increases the mobility of electrons movingparallel to the surface of the wafer, thus increasing the frequency ofoperation of the MOSFET and the associated circuit. Second, the bandoffset between the relaxed SiGe and the tensile Si will confineelectrons in the Si layer. Therefore, in an electron channel device(n-channel), the channel can be removed from the surface or ‘buried’.This ability to spatially separate the charge carriers from scatteringcenters such as ionized impurities and the ‘rough’ oxide interfaceenables the production of low noise, high performance analog devices andcircuits.

A key development in this field was the invention of relaxed SiGebuffers with low threading dislocation densities. The key backgroundinventions in this area are described in U.S. Pat. No. 5,442,205 issuedto Brasen et al. and U.S. Pat. No. 6,107,653 issued to Fitzgerald. Thesepatents define the current best methods of fabricating high qualityrelaxed SiGe.

Novel device structures in research laboratories have been fabricated onearly, primitive versions of the relaxed buffer. For example, strainedSi, surface channel nMOSFETs have been created that show enhancements ofover 60% in intrinsic g_(m) with electron mobility increases of over 75%(Rim et al, IEDM 98 Tech. Dig. p. 707). Strained Si, buried channeldevices demonstrating high transconductance and high mobility have alsobeen fabricated (U. Konig, MRS Symposium Proceedings 533, 3 (1998)).Unfortunately, these devices possess a variety of problems with respectto commercialization. First, the material quality that is generallyavailable is insufficient for practical utilization, since the surfaceof SiGe on Si becomes very rough as the material is relaxed viadislocation introduction. These dislocations are essential in the growthof relaxed SiGe layers on Si since they compensate for the stressinduced by the lattice mismatch between the materials. For more than 10years, researchers have tried to intrinsically control the surfacemorphology through epitaxial growth, but since the stress fields fromthe misfit dislocations affect the growth front, no intrinsic epitaxialsolution is possible. The invention describes a method of planarizationand regrowth that allows all devices on relaxed SiGe to possess asignificantly flatter surface. This reduction in surface roughnessincreases the yield for fine-line lithography, thus enabling themanufacture of strained Si devices.

A second problem with the strained Si devices made to date is thatresearchers have been concentrating on devices optimized for verydifferent applications. The surface channel devices have been exploredto enhance conventional MOSFET devices, whereas the buried channeldevices have been constructed in ways that mimic the buried channeldevices previously available only in III-V materials systems, likeAlGaAs/GaAs. Recognizing that the Si manufacturing infrastructure needsa materials platform that is compatible with Si, scalable, and capableof being used in the plethora of Si integrated circuit applications, thedisclosed invention provides a platform that allows both the enhancementof circuits based on Si CMOS, as well as the fabrication of analogcircuits. Thus, high performance analog or digital systems can bedesigned with this platform. An additional advantage is that both typesof circuits can be fabricated in the CMOS process, and therefore acombined, integrated digital/analog system can be designed as asingle-chip solution.

With these advanced SiGe material platforms, it is now possible toprovide a variety of device and circuit topologies that take advantageof this new materials system. Exemplary embodiments of the inventiondescribe structures and methods to fabricate advanced strained-layer Sidevices, and structures and methods to create circuits based on amultiplicity of devices, all fabricated from the same starting materialplatform. Starting from the same material platform is key to minimizingcost as well as to allowing as many circuit topologies to be built onthis platform as possible.

SUMMARY OF THE INVENTION

Accordingly, the invention provides a material platform of planarizedrelaxed SiGe with regrown device layers. The planarization and regrowthstrategy allows device layers to have minimal surface roughness ascompared to strategies in which device layers are grown withoutplanarization. This planarized and regrown platform is a host forstrained Si devices that can possess optimal characteristics for bothdigital and analog circuits. Structures and processes are described thatallow for the fabrication of high performance digital logic or analogcircuits, but the same structure can be used to host a combination ofdigital and analog circuits, forming a single system-on-chip.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a structure including a relaxedSiGe layer epitaxially grown on a Si substrate,

FIG. 2 is a schematic block diagram of an exemplary structure showingthat the origin of the crosshatch pattern is the stress fields frominjected misfit dislocations;

FIG. 3 is a table showing surface roughness data for relaxed SiGebuffers produced by dislocation injection via graded SiGe layers on Sisubstrates;

FIGS. 4A-4D show an exemplary process flow and resulting platformstructure in accordance with the invention;

FIGS. 5A-5D are schematic diagrams of the corresponding process flow andlayer structure for a surface channel FET platform in accordance withthe invention;

FIGS. 6A-6D are schematic diagrams of the corresponding process flow andlayer structure for a buried channel FET platform in accordance with theinvention;

FIGS. 7A-7D are schematic diagrams of a process flow for a surfacechannel MOSFET in accordance with the invention;

FIGS. 8A and 8B are schematic block diagrams of surface channel deviceswith protective layers;

FIGS. 9A and 9B are schematic block diagrams of surface channel deviceswith Si layers on Ge-rich layers for use in silicide formation;

FIGS. 10 is schematic diagram of a buried channel MOSFET after deviceisolation in accordance with the invention;

FIG. 11 is a schematic flow of the process, for any heterostructure FETdevice deposited on relaxed SiGe, in accordance with the invention;

FIGS. 12A-12D are schematic diagrams of a process flow in the case offorming the surface channel MOSFET in the top strained Si layer inaccordance with the invention;

FIGS. 13A-13D are schematic diagrams of a process flow in the case offorming the surface channel MOSFET in the buried strained Si layer inaccordance with the invention; and

FIGS. 14A and 14B are schematic diagrams of surface and buried channeldevices with Si_(1−y)Ge_(y) channels on a relaxed Si_(1−z)Ge_(z) layer.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic block diagram of a structure 100 including arelaxed SiGe layer epitaxially grown on a Si substrate 102. In thisstructure, a compositionally graded buffer layer 104 is used toaccommodate the lattice mismatch between the uniform SiGe layer 106 andthe Si substrate. By spreading the lattice mismatch over a distance, thegraded buffer minimizes the number of dislocations reaching the surfaceand thus provides a method for growing high-quality relaxed SiGe filmson Si.

Any method of growing a high-quality, relaxed SiGe layer on Si willproduce roughness on the surface of the SiGe layer in a well-knowncrosshatch pattern. This crosshatch pattern is typically a few hundredangstroms thickness over distances of microns. Thus, the crosshatchpattern is a mild, undulating surface morphology with respect to thesize of the electron or hole. For that reason, it is possible to createindividual devices that achieve enhancements over their control Sidevice counterparts. However, commercialization of these devicesrequires injection of the material into the Si CMOS process environmentto achieve low cost, high performance targets. This processingenvironment requires that the material and device characteristics haveminimal impact on the manufacturing process. The crosshatch pattern onthe surface of the wafer is one limiting characteristic of relaxed SiGeon Si that affects the yield and the ease of manufacture. Greaterplanarity is desired for high yield and ease in lithography.

The origin of the crosshatch pattern is the stress fields from theinjected misfit dislocations. This effect is depicted by the exemplarystructure 200 shown in FIG. 2. By definition, the dislocations must beintroduced in order to accommodate the lattice-mismatch between the SiGealloy and the Si substrate. The stress fields originate at thedislocations, and are terminated at the surface of the film. However,the termination at the surface creates crystal lattices that vary fromplace to place on the surface of the wafer. Since growth rate can becorrelated to lattice constant size, different thicknesses of depositionoccur at different points on the wafer. One may think that thick layergrowth beyond the misfit dislocations will smooth the layer of thesethickness differences. Unfortunately, the undulations on the surfacehave a relatively long wavelength; therefore, surface diffusion istypically not great enough to remove the morphology.

FIG. 3 is a table that displays surface roughness data for relaxed SiGebuffers produced by dislocation injection via graded SiGe layers on Sisubstrates. Note that the as-grown crosshatch pattern for relaxedSi_(0.8)Ge_(0.2) buffers creates a typical roughness of approximately7.9 nm. This average roughness increases as the Ge content in therelaxed buffer is increased. Thus, for any SiGe layer that is relaxedthrough dislocation introduction during growth, the surface roughness isunacceptable for state-of-the-art fabrication facilities. After theprocess in which the relaxed SiGe is planarized, the average roughnessis less than 2 nm (typically 0.57 nm), and after device layerdeposition, the average roughness is 0.77 nm with a 1.5 μm regrowththickness. Therefore, after the complete structure is fabricated, overone order of magnitude of roughness reduction can be achieved.

The regrowth device layers can be either greater than or less than thecritical thickness of the regrowth layer. In general, in anylattice-mismatched epitaxial growth, thin layers can be depositedwithout fear of dislocation introduction at the interface. At a greatenough thickness, any lattice-mismatch between the film and substratewill introduce misfit dislocations into the regrown heterostructure.These new dislocations can cause additional surface roughness. Thus, ifthe lattice-mismatch between the regrowth device layers and relaxed SiGebuffer is too great, the effort of planarizing the relaxed SiGe may belost since massive dislocation introduction will roughen the surface.

There are two distinct possibilities with respect to the regrowththickness and the quality of surface. If the regrowth layers are verythin, then exact lattice matching of the regrowth layer composition andthe relaxed buffer composition is not necessary. In this case, thesurface roughness will be very low, approximately equal to thepost-planarization flatness. However, in many applications for devices,the regrowth layer thickness will be 1-2 μm or more. For a 1% differencein Ge concentration between the relaxed SiGe and the regrowth layer, thecritical thickness is approximately 0.5 μm. Thus, if optimal flatness isdesired, it is best to keep the regrowth layer below approximately 0.5μm unless excellent control of the uniformity of Ge concentration acrossthe wafer is achieved. Although this composition matching is achievablein state-of-the-art tools, FIG. 3 shows that less precise matching,i.e., within 2% Ge, results in misfit dislocation introduction andintroduction of a new crosshatch pattern. However, because the latticemismatch is so small, the average roughness is still very low,approximately 0.77 nm. Thus, either lattice-matching or slight mismatchwill result in excellent device layer surfaces for processing.

It is also noted that the relaxed SiGe alloy with surface roughness maynot necessarily be a uniform composition relaxed SiGe layer on a gradedcomposition layer. Although this material layer structure has been shownto be an early example of high quality relaxed SiGe, there are somedisadvantages to this structure. For example, SiGe alloys possess a muchworse coefficient of thermal conductivity than pure Si. Thus; forelectronic devices located at the surface, it may be relativelydifficult to guide the heat away from the device areas due to the thickgraded composition layer and uniform composition layer.

Another exemplary embodiment of the invention, shown in FIGS. 4A-4D,solves this problem and creates a platform for high power SiGe devices.FIGS. 4A-4D show an exemplary process flow and resulting platformstructure in accordance with the invention. The structure is produced byfirst forming a relaxed uniform SiGe alloy 400 via a compositionallygraded layer 402 on a Si substrate 404. The SiGe layer 400 is thentransferred to a second Si substrate 406 using conventional bonding. Forexample, the uniform SiGe alloy 400 on the graded layer 402 can beplanarized to remove the crosshatch pattern, and that relaxed SiGe alloycan be bonded to the Si wafer. The graded layer 402 and the originalsubstrate 404 can be removed by a variety of conventional processes. Forexample, one process is to grind the original Si substrate away andselectively etch to the SiGe, either by a controlled dry or wet etch, orby embedding an etch stop layer. The end result is a relaxed SiGe alloy400 on Si without the thick graded layer. This structure is more suitedfor high power applications since the heat can be conducted away fromthe SiGe layer more efficiently.

The bond and substrate removal technique can also be used to produceSiGe on insulator substrates, or SGOI. An SGOI wafer is produced usingthe same technique shown in FIGS. 4A-4D; however, the second substrateis coated with a SiO₂ layer before bonding. In an alternativeembodiment, both wafers can be coated with SiO₂ to enable oxide-to-oxidebonding. The resulting structure after substrate removal is a highquality, relaxed SiGe layer on an insulating film. Devices built on thisplatform can utilize the performance enhancements of both strained Siand the SOI architecture.

It will be appreciated that in the scenario where the SiGe layer istransferred to another host substrate, one may still need to planarizebefore regrowing the device layer structure. The SiGe surface can be toorough for state of the art processing due to the substrate removaltechnique. In this case, the relaxed SiGe is planarized, and the devicelayers are regrown on top of the high-quality relaxed SiGe surface.

Planarization of the surface via mechanical or other physical methods isrequired to flatten the surface and to achieve CMOS-quality devices.However, the field effect transistors (FETs) that allow for enhanceddigital and analog circuits are very thin, and thus would be removed bythe planarization step. Thus, a first part of the invention is torealize that relaxed SiGe growth and planarization, followed by devicelayer regrowth, is key to creating a high-performance, high yieldenhanced CMOS platform. FIGS. 5 and 6 show the process sequence andregrowth layers required to create embodiments of surface channel andburied channel FETs, respectively.

FIGS. 5A-5D are schematic diagrams of a process flow and resulting layerstructure in accordance with the invention. FIG. 5A shows the surfaceroughness 500, which is typical of a relaxed SiGe alloy 502 on asubstrate 504, as an exaggerated wavy surface. Note that the substrateis labeled in a generic way, since the substrate could itself be Si, arelaxed compositionally graded SiGe layer on Si, or another material inwhich the relaxed SiGe has been transferred through a wafer bonding andremoval technique. The relaxed SiGe alloy 502 is planarized (FIG. 5B) toremove the substantial roughness, and then device regrowth layers 506are epitaxially deposited (FIG. 5C). It is desirable to lattice-matchthe composition of the regrowth layer 506 as closely as possible to therelaxed SiGe 502; however, a small amount of mismatch and dislocationintroduction at the interface is tolerable since the surface remainssubstantially planar. For a surface channel device, a strained Si layer508 of thickness less than 0.1 μm is then grown on top of the relaxedSiGe 502 with an optional sacrificial layer 510, as shown in FIG. 5D.The strained layer 508 is the layer that will be used as the channel inthe final CMOS devices.

FIGS. 6A-6D are schematic diagrams of the corresponding process flow andlayer structure for a buried channel FET platform in accordance with theinvention. In this structure, the regrowth layers 606 include a latticematched SiGe layer 602, a strained Si channel layer 608 with a thicknessof less than 0.05 μm, a SiGe separation or spacer layer 612, a Si gateoxidation layer 614, and an optional sacrificial layer 610 used toprotect the heterostructure during the initial device processing steps.

Once the device structure has been deposited, the rest of the processflow for device fabrication is very similar to that of bulk Si. Asimplified version of the process flow for a surface channel MOSFET inaccordance with the invention is shown in FIGS. 7A-7D. This surfacechannel MOSFET contains a relaxed SiGe layer 700 and a strained Si layer702. The device isolation oxide 704, depicted in FIG. 7A, is typicallyformed first. In this step, the SiN layer 706, which is on top of a thinpad oxide layer 708, serves as a hard mask for either local oxidation ofsilicon (LOCOS) or shallow trench isolation (STI). Both techniques use athick oxide (relative to device dimensions) to provide a high thresholdvoltage between devices; however, STI is better suited forsub-quarter-micron technologies. FIG. 7B is a schematic of the devicearea after the gate oxide 716 growth and the shallow-source drainimplant. The implant regions 710 are self-aligned by using a poly-Sigate 712 patterned with photoresist 714 as a masking layer.Subsequently, deep source-drain implants 718 are positioned usingconventional spacer 720 formation and the device is electricallycontacted through the formation of silicide 722 at the gate andsilicide/germanides 724 at the source and drain (FIG. 7C). FIG. 7D is aschematic of the device after the first level of metal interconnects 726have been deposited and etched.

Since there are limited-thickness layers on top of the entire structure,the removal of surface material during processing becomes more criticalthan with standard Si. For surface channel devices, the structure thatis regrown consists primarily of nearly lattice-matched SiGe, and a thinsurface layer of strained Si. Many of the processes that are at thebeginning of a Si fabrication sequence strip Si from the surface. If theprocessing is not carefully controlled, the entire strained Si layer canbe removed before the gate oxidation. The resulting device will be arelaxed SiGe channel FET and thus the benefits of a strained Si channelwill not be realized.

A logical solution to combat Si removal during initial processing is tomake the strained Si layer thick enough to compensate for this removal.However, thick Si layers are not possible for two reasons. First, theenhanced electrical properties originate from the fact that the Si isstrained and thick layers experience strain relief through theintroduction of misfit dislocations. Second, the misfit dislocationsthemselves are undesirable in significant quantity, since they canscatter carriers and increase leakage currents in junctions.

In order to prevent removal of strained Si layers at the surface, thecleaning procedures before gate oxidation must be minimized and/orprotective layers must be applied. Protective layers are useful sincetheir removal can be carefully controlled. Some examples of protectivelayers for surface channel devices are shown in FIGS. 8A and 8B. FIG. 8Ashows a strained Si heterostructure of a relaxed SiGe layer 800 and astrained Si channel layer 802 protected by a surface layer 804 of SiGe.The surface SiGe layer 804 should have a Ge concentration similar tothat of the relaxed SiGe layer 800 below, so that the thickness is notlimited by critical thickness constraints. During the initial cleans,the SiGe sacrificial layer is removed instead of the strained Si channellayer. The thickness of the sacrificial layer can either be tuned toequal the removal thickness, or can be made greater than the removalthickness. In the latter case, the excess SiGe can be selectivelyremoved before the gate oxidation step to reveal a clean, strained Silayer at the as grown thickness. If the particular fabrication facilityprefers a Si terminated surface, a sacrificial Si layer may be depositedon top of the SiGe sacrificial cap layer.

FIG. 8B shows a structure where a layer 806 of SiO₂ and a surface layer808 of either a poly-crystalline or an amorphous material are used asprotective layers. In this method, an oxide layer is either grown ordeposited after the epitaxial growth of the strained Si layer.Subsequently, a polycrystalline or amorphous layer of Si, SiGe, or Ge isdeposited. These semiconductor layers protect the strained-Si layer inthe same manner as a SiGe cap during the processing steps before gateoxidation. Prior to gate oxidation, the poly/amorphous and oxide layersare selectively removed. Although the sacrificial layers are shown asprotection for a surface channel device, the same techniques can beemployed in a buried channel heterostructure.

Another way in which conventional Si processing is modified is duringthe source-drain silicide-germanide formation (FIG. 7C). In conventionalSi processing, a metal (typically Ti, Co, or Ni) is reacted with the Siand, through standard annealing sequences, low resistivity silicides areformed. However, in this case, the metal reacts with both Si and Gesimultaneously. Since the suicides have much lower free energy than thegermanides, there is a tendency to form a silicide while the Ge isexpelled. The expelled germanium creates agglomeration and increases theresistance of the contacts. This increase in series resistance offsetsthe benefits of the extra drive current from the heterostructure, andnegates the advantages of the structure.

Ti and Ni can form phases in which the Ge is not rejected severely, thusallowing the formation of a good contact. Co is much more problematic.However, as discussed above for the problem of Si removal, a protectivelayer(s) at the device epitaxy stage can be applied instead ofoptimizing the SiGe-metal reaction. For example, the strained Si thatwill become the surface channel can be coated with a high-Ge-contentSiGe alloy (higher Ge content than the initial relaxed SiGe), followedby strained Si. Two approaches are possible using these surface contactlayers. Both methods introduce thick Si at the surface and allow theconventional silicide technology to be practiced without encounteringthe problems with SiGe-metal reactions.

The first approach, shown on a surface channel heterostructure 900 inFIG. 9A, uses a Ge-rich layer 906 thin enough that it is substantiallystrained. The layer 906 is provided on a strained Si channel layer 904and relaxed SiGe layer 902. In this case, if a subsequent Si layer 908is beyond the critical thickness, the compressive Ge-rich layer 906 actsas a barrier to dislocations entering the strained Si channel 904. Thisbarrier is beneficial since dislocations do not adversely affect thesilicide process; thus, their presence in the subsequent Si layer 908 isof no consequence. However, if the dislocations were to penetrate to thechannel, there would be adverse effects on the device.

A second approach, shown in FIG. 9B, is to allow a Ge-rich layer 910 tointentionally exceed the critical thickness, thereby causing substantialrelaxation in the Ge-rich layer. In this scenario, an arbitrarily thickSi layer 912 can be applied on top of the relaxed Ge-rich layer. Thislayer will contain more defects than the strained channel, but thedefects play no role in device operation since this Si is relevant onlyin the silicide reaction. In both cases, the process is free from themetal-SiGe reaction concerns, since the metal will react with Si-only.Once the silicide contacts have been formed, the rest of the sequence isa standard Si CMOS process flow, except that the thermal budget iscarefully monitored since, for example, the silicide-germanicide (ifthat option is used) typically cannot tolerate as high a temperature asthe conventional silicide. A major advantage of using Si/SiGe FETheterostructures to achieve enhanced performance is the compatibilitywith conventional Si techniques. Many of the processes are identical toSi CMOS processing, and once the front-end of the process, i.e., theprocessing of the Si/SiGe heterostructure, is complete, the entireback-end process is uninfluenced by the fact that Si/SiGe lies below.

Even though the starting heterostructure for the buried channel deviceis different from that of the surface channel device, its process flowis very similar to the surface channel flow shown in FIGS. 7A-7D. FIG.10 is a schematic block diagram of a buried channel MOSFET structure1000 after the device isolation oxide 1016 has been formed using a SiNmask 1014. In this case, the strained channel 1002 on a first SiGe layer1010 is separated from the surface by the growth of another SiGe layer1004, followed by another Si layer 1006. This Si layer is needed for thegate oxide 1008 since gate-oxide formation on SiGe produces a very highinterface state density, thus creating non-ideal MOSFETs. Oneconsequence of this Si layer, is that if it is too thick, a substantialportion of the Si layer will remain after the gate oxidation. Carrierscan populate this residual Si layer, creating a surface channel inparallel with the desired buried channel and leading to deleteriousdevice properties. Thus, the surface layer Si must be kept as thin aspossible, typically less than 50 Å and ideally in the range of 5-15 Å.

Another added feature that is necessary for a buried channel device isthe supply layer implant. The field experienced in the verticaldirection when the device is turned on is strong enough to pull carriersfrom the buried channel 1002 and force them to populate a Si channel1006 near the Si/SiO₂ interface 1012, thus destroying any advantage ofthe buried channel. Thus, a supply layer of dopant must be introducedeither in the layer 1004 between the buried channel and the top Si layer1006, or below the buried channel in the underlying SiGe 1010. In thisway, the device is forced on with little or no applied voltage, andturned off by applying a voltage (depletion mode device).

FIG. 11 is a schematic flow of the process, for any heterostructure FETdevice deposited on relaxed SiGe, in accordance with the invention. Themain process steps are shown in the boxes, and optional steps orcomments are shown in the circles. The first three steps(1100,1102,1104) describe the fabrication of the strained siliconheterostructure. The sequence includes production of relaxed SiGe on Si,planarization of the SiGe, and regrowth of the device layers. Once thestrained heterostructure is complete (1106), MOS fabrication begins withdevice isolation (1112) using either STI (1110) or LOCOS (1108). Beforeproceeding to the gate oxidation, buried channel devices undergo asupply and threshold implant (1114), and any protective layers appliedto either a buried or surface channel heterostructure must beselectively removed (1116). The processing sequence after the gateoxidation (1118) is similar to conventional Si CMOS processing. Thesesteps include gate deposition, doping, and definition (1120),self-aligned shallow source-drain implant (1122), spacer formation(1124), self-aligned deep source-drain implant (1126), salicideformation (1128), and pad isolation via metal deposition and etch(1130). The steps requiring significant alteration have been discussed.

One particular advantage of the process of FIG. 11 is that it enablesthe use of surface channel and buried channel devices on the sameplatform. Consider FIGS. 12A-12D and FIGS. 13A-13D, which show auniversal substrate layer configuration and a process that leads to theco-habitation of surface and buried channel MOSFETs on the same chip.The universal substrate is one in which both surface channel and buriedchannel devices can be fabricated. There are two possibilities infabricating the surface channel device in this sequence, shown in FIGS.12 and 13. The process flows for combining surface and buried channelare similar to the previous process described in FIG. 7. Therefore, onlythe critical steps involved in exposing the proper gate areas are shownin FIGS. 12 and 13.

FIGS. 12A and 13A depict the same basic heterostructure 1200,1300 forintegrating surface channel and buried channel devices. There is asurface strained Si layer 1202,1302, a SiGe spacer layer 1204,1304, aburied strained Si layer 1206,1306, and a relaxed platform of SiGe1208,1308. Two strained Si layers are necessary because the buriedchannel MOSFET requires a surface Si layer to form the gate oxide and aburied Si layer to form the device channel. The figures also show adevice isolation region 1210 that separates the buried channel devicearea 1212,1312 from the surface channel device area 1214,1314.

Unlike the buried channel device, a surface channel MOSFET only requiresone strained Si layer. As a result, the surface channel MOSFET can befabricated either in the top strained Si layer, as shown in FIGS.12B-12D, or the buried Si layer channel, as shown in FIGS. 13B-13D. FIG.12B is a schematic diagram of a surface channel gate oxidation 1216 inthe top Si layer 1202. In this scenario, a thicker top Si layer isdesired, since after oxidation, a to residual strained Si layer must bepresent to form the channel. FIG. 12B also shows a possible position forthe buried channel supply implant 1218, which is usually implantedbefore the buried channel gate oxide is grown. Since the top Si layer isoptimized for the surface channel device, it may be necessary to stripsome of the top strained Si in the regions 1220 where buried channeldevices are being created, as shown in FIG. 12C. This removal isnecessary in order to minimize the surface Si thickness after gate oxide1222 formation (FIG. 12D), and thus avoid the formation of a paralleldevice channel.

When a surface channel MOSFET is formed in the buried strained Si layer,the top strained Si layer can be thin, i.e., designed optimally for theburied channel MOSFET. In FIG. 13B, the top strained Si and SiGe layersare removed in the region 1312 where the surface channel MOSFETs areformed. Because Si and SiGe have different properties, a range ofselective removal techniques can be used, such as wet or dry chemicaletching. Selective oxidation can also be used since SiGe oxidizes atmuch higher rates than Si, especially under wet oxidation conditions.FIG. 13C shows the gate oxidation 1314 of the surface channel device aswell as the supply layer implant 1316 for the buried channel device.Finally, FIG. 13D shows the position of the buried channel gate oxide1318. No thinning of the top Si layer is required prior to the oxidationsince the epitaxial thickness is optimized for the buried channeldevice. Subsequent to these initial steps, the processing for eachdevice proceeds as previously described.

Another key step in the process is the use of a localized implant tocreate the supply layer needed in the buried channel device. In a MOSFETstructure, when the channel is turned on, large vertical fields arepresent that bring carriers to the surface. The band offset between theSi and SiGe that confines the electrons in the buried strained Si layeris not large enough to prevent carriers from being pulled out of theburied channel. Thus, at first, the buried channel MOSFET would appearuseless. However, if enough charge were present in the top SiGe layer,the MOSFET would become a depletion-mode device, i.e. normally on andrequiring bias to turn off the channel. In the surface/buried channeldevice platform, a supply layer implant can be created in the regionswhere the buried channel will be fabricated, thus easing processintegration. If for some reason the supply layer implant is notpossible, note that the process shown in FIG. 11 in which the surfacechannel is created on the buried Si layer is an acceptable process,since the dopant can be introduced into the top SiGe layer duringepitaxial growth. The supply layer is then removed from the surfacechannel MOSFET areas when the top SiGe and strained Si layers areselectively etched away.

In the processes described in FIGS. 10, 12 and 13, it is assumed thatthe desire is to fabricate a buried channel MOSFET. If the oxide of theburied channel device is removed, one can form a buried channel devicewith a metal gate (termed a MODFET or HEMT). The advantage of thisdevice is that the transconductance can be much higher since there is adecrease in capacitance due to the missing oxide. However, there are twodisadvantages to using this device. First, all thermal processes aftergate definition have to be extremely low temperature, otherwise themetal will react with the semiconductor, forming an alloyed gate with avery low, or non-existent, barrier. Related to this issue is the seconddisadvantage. Due to the low thermal budget, the source and drainformation and contacts are typically done before the gate definition.Inverting these steps prevents the gate from being self-aligned to thesource and drain, thus increasing the series resistance between the gateand the source and drain. Therefore, with a carefully designed buriedchannel MOSFET, the self-aligned nature can be a great advantage indevice performance. Another benefit of the MOSFET structure is that thegate leakage is very low.

The combination of buried n-channel structures with n and p type surfacechannel MOSFETs has been emphasized heretofore. It is important to alsoemphasize that in buried n-channel devices as well as in surface channeldevices, the channels need not be pure Si. Si_(1−y)Ge_(y) channels canbe used to increase the stability during processing. FIGS. 14A and 14Bare schematic diagrams of surface 1400 and buried 1450 channel deviceswith Si_(1−y)Ge_(y) channels 1402 on a relaxed Si_(1−z)Ge_(z) layer1404. The devices are shown after salicidation and thus contain apoly-Si gate 1410, gate oxide 1408, silicide regions 1412, spacers 1414,and doped regions 1416. In the surface channel device 1400, a thin layer1406 of Si must be deposited onto the Si_(1−y)Ge_(y) layer 1402 to formthe gate oxide 1408, as previously described for buried channel devices.In the buried Si_(1−y)Ge_(y) channel device 1450, the device layersequence is unchanged and consists of a buried strained channel 1402, aSiGe spacer layer 1418, and a surface Si layer 1420 for oxidation.

To maintain tensile strain in the channel of an nMOS device, the latticeconstant of the channel layer must be less than that of the relaxed SiGelayer, i.e., y must be less than z. Since n-channel devices aresensitive to alloy scattering, the highest mobilities result when theconcentration in the channel is low. In order to have strain on thischannel layer at a reasonable critical thickness, the underlying SiGeshould have a Ge concentration in the range of 10-50%.

Experimental data indicates that p channels are less sensitive to alloyscattering. Thus, surface MOSFETs with alloy channels are also possible.In addition, the buried channel devices can be p-channel devices simplyby having the Ge concentration in the channel, y, greater than the Geconcentration in the relaxed SiGe alloy, z, and by switching the supplydopant from n-type to p-type. This configuration can be used to form Gechannel devices when y=1 and 0.5<z<0.9.

With the ability to mix enhancement mode surface channel devices (n andp channel, through implants as in typical Si CMOS technology) anddepletion-mode buried channel MOSFETs and MODFETs, it is possible tocreate highly integrated digital/analog systems. The enhancement modedevices can be fabricated into high performance CMOS, and the regions ofan analog circuit requiring the high performance low-noise depletionmode device can be fabricated in the buried channel regions. Thus, it ispossible to construct optimal communication stages, digital processingstages, etc. on a single platform. These different regions are connectedelectrically in the backend of the Si CMOS chip, just as transistors areconnected by the back-end technology today. Thus, the only changes tothe CMOS process are some parameters in the processes in the fabricationfacility, and the new material, but otherwise, the entire manufacturingprocess is transparent to the change. Thus, the economics favor such aplatform for integrated Si CMOS systems on chip.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A semiconductor structure integrating separatesurface channel and buried channel devices, the structure comprising: a.a first strained layer; b. a second strained layer thereunder; c. anintervening spacer layer between and adjacent the first strained layerand the second strained layer; d. a surface-channel device having acarrier channel in the first strained layer; and e. a buried-channeldevice having a carrier channel in the second strained layer.
 2. Thestructure of claim 1, wherein the at least one surface-channel device isa MOSFET comprising a gate dielectric over the first strained layer. 3.The structure of claim 1, wherein the at least one buried-channel deviceis a FET comprising a thinned portion of the first strained layer and agate dielectric thereover.
 4. The structure of claim 3, whereinburied-channel device comprises a dopant supply layer in a portion ofthe spacer layer adjacent the thinned portion of the first strainedlayer.
 5. The structure of claim 3, wherein the buried-channel devicecomprises a dopant supply layer below and adjacent the second strainedlayer.
 6. The structure of claim 1, further comprising a platform layeradjacent the second strained layer.
 7. The structure of claim 6, whereinthe platform layer comprises at least one planarized surface.
 8. Thestructure of claim 1, wherein the spacer layer is a relaxed compositionof silicon and germanium.
 9. The structure of claim 1, wherein at leastone of the strained layers is silicon.
 10. The structure of claim 1,wherein both of the strained layers are silicon.
 11. The structure ofclaim 1, wherein at least the first surface strained layer comprises Ge.12. The structure of claim 1, wherein at least the second strained layercomprises Ge.
 13. The structure of claim 1, wherein the second strainedlayer is compressively strained.
 14. The structure of claim 1, whereinthe second strained layer is tensilely strained.
 15. The structure ofclaim 1, wherein a carrier channel of a surface-channel device isprovided in the second strained layer.
 16. The structure of claim 1,further comprising a substrate adjacent the second strained layer. 17.The structure of claim 18, wherein the substrate comprises at least oneplanarized surface.
 18. The structure of claim 16, further comprising aninsulator disposed between the substrate and the second strained layer.19. The structure of claim 18, wherein the insulator is SiO₂.